Assessment of Pain in Animals

The assessment of pain is of critical importance for mechanistic studies as well as for the validation of drug targets. The study of pain in awake animals raises ethical, philosophical, and technical problems. Philosophically, there is the problem that pain cannot be monitored directly in animals but can only be estimated by examining their responses to nociceptive stimuli; however, such responses do not necessarily mean that there is a concomitant sensation. In this chapter, I highlight several types of nociceptive stimuli (thermal, mechanical, or chemical), which have been used in different pain models such as acute pain, chronic pain, arthritis pain, inflammatory, and visceral pain. The monitored reactions are almost always motor responses ranging from spinal reflexes to complex behaviors. Most have the weakness that they may be associated with, or modulated by, other physiological functions. The main methods are reviewed in terms of their sensitivity, specificity, and predictiveness. Although the neural basis of the most commonly used tests is poorly understood, their use will be more profitable if pain is considered within the framework of, rather than apart from, the body’s homeostatic mechanisms.